Giant electric field-induced strains in lead-free ceramics

[Editor’s note: This report comes to us from the laboratory of Xiaoli Tan, PhD, professor in the Department of Materials Science & Engineering at Iowa State University. Xiaoming Liu is a graduate student in Tan’s lab.]

by Xiaoming Liu

Lead-containing ceramics of Pb(ZrxTi1-x)O3 (PZT) have been widely used in commercial applications due to their excellent electrical properties, including a large strain response with respect to an applied electric field, which benefits actuator devices.

In the past decade, however, significant environmental concerns surrounding lead contamination have led to the development of lead-free functional materials in an effort to replace lead-containing solid solutions with nontoxic materials that have similar properties.

According to previous work, lead-free single crystals show giant electric field-induced strain (i.e., electrostrain) close to 1.0%. Nevertheless, high fabrication costs limit these materials’ applications.

(K,Na)NbO3-based, (Bi1/2Na1/2)TiO3-based, and BaTiO3-based compounds synthesized with conventional methods can have large strains, but they never exceed 0.5%. In addition, lead-free polycrystalline ceramics are intrinsically brittle and tend to fracture with repeated application of electric fields.

An Iowa State University research team led by Xiaoli Tan designs lead-free ceramic compositions and optimizes processing conditions for ultrahigh electrostrains. The team’s latest work—which builds upon a previous CTT report from the same group describing the piezoelectric mechanisms in lead-free ferroelectrics—shows that high electrostrains can be developed in (Bi1/2Na1/2)TiO3-based compositions due primarily to reversible electric field-induced phase transitions.

The Tan lab’s recent publication in Advanced Materialsshows that Sr and Nb co-doped [Bi1/2(Na,K)1/2]TiO3 (abbreviated BNT-Nb) polycrystalline ceramics can generate a giant electrostrain of 0.70% with a corresponding large-signal piezoelectric coefficient (d33* = 1400 pm/V). This strain value is the highest reported to date in any lead-free polycrystalline ceramic and represents a 50% improvement over previous results. Further, the ceramic pellet remains a single piece even after 100 electrical cycles.

In piezoelectric ceramics, volume deformation of bulk materials correlates to domain alignment and domain switching under applied electric fields. In lead-free compositions, electrostrain is often contributed by phase transitions. The Tan group took advantage of in-situ transmission electron microscopy to study crystal structure evolution under the electric field of this unique material to confirm that reversible phase transition is indeed the main reason for the material’s giant electrostrain (see figure below).

Credit: Tan lab

According to the Advanced Materials article, “Such a high strain is originated from phase transitions between the ergodic relaxor phases in the form of mixed R3c and P4bm nanometer-sized domains and the ferroelectric R3c phase in the form of lamellar domains.”

This work highlights the giant electrostrain and high d33* value of BNT-Nb. In addition, ultrahigh strain still maintains values greater than 0.5% after 100 electrical cycles (see figure above). This discovery suggests that BNT-Nb ceramics are extremely promising for actuator applications and will stimulate new research on fracture-resistant deformable ceramics for structural applications.